The occurrence and reactivity of phenoxyl linkages in lignin and low rank coal

Nowadays crude oil and natural gas are the main sources for the production of fuels and feedstock chemicals. The resources are limited and the application of (renewable) alternatives will be needed in the future to sustain the progress of the global community. Coal is a clear option since the reserves are much larger than those of oil. Currently, the main use of coal is as a source of energy by direct combustion. A renewed interest exists in the production of petrochemicals and synthetic fuels by means of liquefaction of coal. The processes are a viable, although to date not economical, option [F.J. Derbyshire, D. Gray, Ullmann's Encyclopedia of industrial Chemistry, 5th Ed., Vol. A7, VCH Verlagsgesellschaft, Weinheim, 1986, p. 197]. Coal is considered to originate from natural waste material (biomass). Biopolymers (cellulose, lignin, polypeptides) are fed into the soil, and along a geological time scale a complicated chemical interconversion follows in which the oxygen content is gradually reduced. These chemical transformations are brought about by means of bacteria and in a later stage by a combination of high pressures and temperatures. With an annual global production of around 172 billion metric tons [A. Heredia, A. Jimenez, R. Guillen, Z. Lebensm, Unters. Forsch. 200 (1995) 24], energy production from biomass seems to be an attractive option. The source is renewable and the atmospheric carbon dioxide is recycled. However, its use for feedstock production demands for a sequence of chemical conversions since the main building blocks in biomass are carbohydrates. Another constituent is lignin, which is removed from wood meal by the pulp and paper industry. In the USA alone about 50 million metric tons of lignin are produced each year and this waste stream is at present mainly burnt. However, in view of its chemical composition, lignin may serve as an interesting basic material for aromatic (phenolic) compounds with a high added value. The first part of this review will deal with lignin: its formation, structure and degradation under thermal and radiative conditions. The second part highlights the structure of coal and the mechanistic features of coal liquefaction. Phenoxyl linkages are important structural elements in both lignin and coal. In contrast to the chemistry of hydrocarbons under thermal liquefaction conditions, the fate of oxygen containing model compounds for lignin and coal is less well understood and will be surveyed in the third part of this review. A final section will deal with the experimental procedures to measure bond dissociation enthalpies (BDEs) and the thermochemical aspects of the phenoxyl linkages, including the effect of substitution. (C) 2000 Elsevier Science B.V. All rights reserved.